Optimized Caliber

[JASmith]

...the case width limits the maximum safe pressure to around 50,000 psi, which I suspect isn't quite enough to extract the performance required to replace the 7.62mm. Particularly when the US Army's preference for lead-free bullets is taken into account (longer bullets = less room for propellant). So all of the proposals below exceed the overall length of the 5.56x45...

I like the photos!

There seems to be continuing confusion, however, about the pressure limitations of the Grendel cartridge.

Per Bill Alexander, the base width indeed places more thrust on the AR15 bolt, which results in reduced average bolt life when run at pressures significantly above 50,000 psi. This is a reliability issue unique to the AR15 bolt.

Also, Bill indicates that Grendel case itself can readily handle upwards of 58000 psi.

These pressures are safe in appropriate actions. When coupled with a longer COAL as suggested, then the desired velocities and bullet weights for supplanting the 7.62X51 M80 cartridge are likely attainable.

[Tony Williams]

I think that to have one cartridge in two versions, differing only in the length of the bullet protrusion and the chamber pressure, would be confusing. We don't want to get into a 6.8mm Rem situation, with two different chambers!

If we're no longer limited to 5.56mm action lengths, then IMO it's better to produce a slightly different case, extending it a bit to make sure that it can still deliver the required performance from 20 inch barrels when loaded with lead-free bullets (especially the even longer tracer versions). That might also mean that it needn't be loaded to maximum pressure to start with, which is no bad thing.

Edit to add: a little extra capacity would also make it easier in due course to produce a polymer cased (with metal base) version, since these tend to have thicker walls. These cases are improving all the time, and offer big weight savings - important for the military.

[stanc]

Some thoughts on the options for a GPC (General Purpose Cartridge) which could replace the 7.62x51 in portable small arms carried by dismounted infantry with substantial reductions in weight and recoil (and could also replace the 5.56mm in the infantry squad, although that would stick around for a long time in non-infantry applications).

I start with the assumption that the ideal GPC cannot be accommodated in the AR-15 platform.

The three versions show 6.35mm, 6.5mm and 6.8mm calibres. These were chosen because the ARDEC calibre survey tested cartridges in 6.35mm and 6.8mm (but not 6.5mm) and concluded that these two were both better than 5.56mm, 6mm or 7.62mm.

There is obviously room to extend the OAL to accommodate lead-free bullets.

Comments?

Looks good. Ought to work.

The 0.42" case offers a slight advantage to machine gun belt length (for 100 linked rounds).

The 0.44" case gives a small advantage to rifle magazine ergonomics (front-to-rear dimension).

No significant weight difference between the two case options. Probably no significant difference in muzzle velocities, either.

Major drawback to both is the non-availability of platforms optimized for intermediate-length, intermediate-diameter cartridges, which means it'd be necessary to use the bulkier, heavier weapons developed for 7.62x51 (such as the AR-10, Mk17, etc).

[Tony Williams]

Major drawback to both is the non-availability of platforms optimized for intermediate-length, intermediate-diameter cartridges, which means it'd be necessary to use the bulkier, heavier weapons developed for 7.62x51 (such as the AR-10, Mk17, etc).

True, development and initial use would be in AR-10 sized actions (although these could maybe be lightened a bit). But if such a round were ever adopted by the military, intermediate sized actions would appear...

[stanc]

True, development and initial use would be in AR-10 sized actions (although these could maybe be lightened a bit).

How can that be done?

[JASmith]

I think that to have one cartridge in two versions, differing only in the length of the bullet protrusion and the chamber pressure, would be confusing. We don't want to get into a 6.8mm Rem situation, with two different chambers! ...

I understand the logic, but there is also the logic of the enormous investment needed to qualify a new cartridge. The 6.8 and the Grendel have both gone through the process. The Grendel even has steel cases in the pipeline.

I've done the math, and adding, say .05" to the body length adds only a grain or two to the case capacity. This is somewhat less than the 3+ grains one gets by merely seating a COAL of 2.4" to 2.5" with no other change to the cartridge. The case remains the same.

I've also looked at scaled drawings of the longer case -- there won't be any confusion between the two because one will have a conventional appearance and the other will have the tip of the bullet sticking waay out there!

Having said the above, I would concur that, should the investment be available (a truly huge IF!), then the most attractive options would boil down to lengthened versions of either the 6.8 or the Grendel, both in 6.5 caliber.

The fatter case of the Grendel-based option would likely be more weight efficient because the wall thicknesses tend to remain about the same, so the case weight in that area goes with the radius, while the volume of the interior goes with the square of the radius.

[LRRPF52]

We would be able to achieve all that we wanted for a "beat 7.62 M80 ball" using a 2.300" COAL and 130gr bullets in the Grendel, launched from a dedicated platform built around the cartridge. There is zero need to develop a new case, when we look at the pressure capacity that the Grendel brass can sustain. We also don't have to ramp the pressure up too crazy, due to the efficiency of 6.5mm projectiles.

[stanc]

We would be able to achieve all that we wanted for a "beat 7.62 M80 ball" using a 2.300" COAL and 130gr bullets in the Grendel...

Question is, can that level of performance be achieved with lighter (110-115gr), lead-free bullets?

[Variable]

Well, if you guys are talking about all new stuff:

1.) Use a new slightly larger magwell like LWRC was/is going to make.

2.) Develop a new uber polymer mag for said magwell.

3.) Use a beefier barrel extension with a larger bolt for higher acceptable pressures.

4.) Stick a fork in it and call it done.

Didn't the .222 spawn the eventual 5.56 cartridge?

The Grendel could spawn a slightly zippier .308 killer if called upon I guess. You'd hear whining from all of us folks with Grendels, but they'd sell even more rifles as we quietly stood in line for backorders on the new "_______"(insert roided Grendel Max cartridge's name here).

We love how the Grendel fits in the AR platform now, but most people eventually buy or build a whole 'nother rifle anyway.

Platform difference:

New upper, barrel extension, bolt, bolt carrier, lower receiver, and magazines.

You could still keep a lot of AR parts, and if somebody like Magpul made the mags then people would fight over who got the first shipment of them....

The first write up in a magazine showing how an 18" carbine performed at 1000 yards would incite a stampede of folk trying to get their hands on the new 6.5 "Death Wand".LOL

[Tony Williams]

How can that be done?

The guns taking part in the current US Army carbine competition have to meet a tight weight limit (7 lbs or so) and have been pared down as a result. I have examined and handled the HK candidate, and although based on the HK 416 it is significantly lighter.

I presume therefore that current 7.62mm weapons could be similarly pared down, especially to handle a smaller and less powerful cartridge.

[stanc]

I don't know if that presumption is warranted. At least, I don't see where any weight can be trimmed from a design that's already rather lean.

[Tony Williams]

Let me put it this way: I have little doubt that the HK417 could be pared down in the same way as the HK416.

The FN MK17 may be less easy to change, but there's hardly any need because it's so light anyway (not much heavier than the MK16).

[LRRPF52]

You can make an AR10 as light as a SCAR-17 by simply using a pencil barrel and lighter COTS components.

If someone showed up to the competition with a DMR system that was adaptable to both 5.56 and Grendel, and they demonstrated what a Grendel could do versus a 7.62 NATO carbine at distance, that could be an eye-opener, especially when considering soldier's load and the ability to sustain the fight longer than 7.62 NATO, with less recoil, and better terminal ballistics.

If I were selling it, I would say,

"Look, 7.62 NATO will be around for a long time, and there is no need to get rid of it in aircraft and vehcile-mounted, mechanically-fired weapons. However, I think we're really at a stage where we can seriously meet some of these constant weight-reduction requirements for the dismounted soldier, by displacing belt-fed 7.62 GPMG's, and DMR's, with a more capable cartidge, in a lighter system. Watch what it does across the target array on this range...."

[JASmith]

Well said!!!

[stanc]

You can make an AR10 as light as a SCAR-17 by simply using a pencil barrel and lighter COTS components.

Yes, but that would not be representative of the weight when configured for military use.

For instance, the SR25 EMC weighs 8.3 lbs.

If someone showed up to the competition with a DMR system that was adaptable to both 5.56 and Grendel, and they demonstrated what a Grendel could do versus a 7.62 NATO carbine at distance, that could be an eye-opener, especially when considering soldier's load and the ability to sustain the fight longer than 7.62 NATO, with less recoil, and better terminal ballistics.

If I were selling it, I would say,

"...I think we're really at a stage where we can seriously meet some of these constant weight-reduction requirements for the dismounted soldier, by displacing belt-fed 7.62 GPMG's, and DMR's, with a more capable cartidge, in a lighter system. Watch what it does across the target array on this range...."

I wholeheartedly agree the concept needs to be demonstrated.

The question is, which approach would give best overall results?

a. Your's - 5.56 NATO for carbine, 6.5 Super G for LMG, DMR.

b. John's - 6.5 Grendel for carbine, LMG, DMR.

c. Tony's - 6.35/6.5/6.8 GPC for rifle, GPMG, DMR.

[Variable]

Yes, but that would not be representative of the weight when configured for military use.

For instance, the SR25 EMC weighs 8.3 lbs.



I wholeheartedly agree the concept needs to be demonstrated.

The question is, which approach would give best overall results?

a. Your's - 5.56 NATO for carbine, 6.5 Super G for LMG, DMR.

b. John's - 6.5 Grendel for carbine, LMG, DMR.

c. Tony's - 6.35/6.5/6.8 GPC for rifle, GPMG, DMR.

Without a wealthy backer with cnc machines, my vote would be "B". Super Grendel would probably be best, but what doesn't exist can't be demonstrated.

Everything is a series of compromises in one form or another, and while I'd like to see a "Super Grendel", I think the current form is sufficient for use across the board.

ETA: I should say that what LRRPF52 said about 5.56 for carbines is spot on. But considering what kind of enviro hoops they are probably going to put the 5.56 ammo through--- I'd just say the heck with it and go Grendel to retain lethality. From a logistics standpoint it would be rather advantageous for one cartridge to cover it all. I'd be willing to take the hit on overall loadout quantity in exchange for increased lethality and penetration from short barrels at various ranges, not just up close.

[LRRPF52]

The SR25 EMC doesn't have the extreme lightweight barrel profile that the SCAR does, and has more 1913 real estate that makes up for more weight than the SCAR carbine-length receiver, which is being lengthened for the SCAR Sniper Support Weapon in at least the 5.56 version. They're not going to get any groups wothwhile in a lightweight SCAR barrel for the SSR, but it does offer a nice heavy-hitting carbine with the -17 for general purpose use augmenting the majority of guys carrying 5.56 carbines.

They could have a lighter gun, with a heavier profile barrel that will provide both the increased terminal energy, and accuracy of a DMR in one shebang with a Grendel DMR, plus not lose any significant effectiveness as a general purpose infantry rifle in the 50m-500m fight.

Combining the introduction of a Grendel LMG, DMR, and the .338 NM MMG could offer the systemic 1-2-3 punch that would be needed to convince the long-term strategists, logisticians, and commanders that we're finally ready to begin the necessary steps to sending 7.62 NATO into its final phase-out schedule, as modular systems like the Mk48 are upgraded with a Grendel Conversion kit, so units can burn the crap out of some 7.62 in training, then phase in Grendel and the .338 MMG powerhouse. With the addition of lightweight, precision-guided, man-portable, indirect-fire systems, the following force structure begins to emerge:

Light Weapons:
Grendel LMG
Grendel DMR

Heavy Weapons:
.338 MMG
LMPPGIFS (Lightweight Mortar w/precision warhead array)

[stanc]

From a logistics standpoint it would be rather advantageous for one cartridge to cover it all. I'd be willing to take the hit on overall loadout quantity in exchange for increased lethality and penetration from short barrels at various ranges, not just up close.

Those arguments are also put forth for the 6.X GPC (and Murray's 7 UIAC). The main difference is in the size and weight of the weapons. The GPC/UIAC require guns as big and heavy as for 7.62 NATO, whereas Grendel fits the smaller, lighter firearms made for 5.56 NATO.

Both of the above would completely replace one NATO cartridge (5.56), and so may be a better fit to the Army way than Paul's 6.5 Super Grendel, which would require adding a third caliber to the inventory alongside 5.56 and 7.62 NATO. From a purely technical standpoint, the Super G seems very attractive, but IMO it has the least chance of success. Plus, as you noted, one cannot demonstrate something that does not exist.

[JASmith]

...6.5 Super Grendel, which would require adding a third caliber to the inventory alongside 5.56 and 7.62 NATO...

The "Super Grendel" would replace the 7.62x51 in dismounted infantry operations. He would dispose of the 7.62x51 in the following manner:

Originally posted by LRRPF52

If I were selling it, I would say,

"Look, 7.62 NATO will be around for a long time, and there is no need to get rid of it in aircraft and vehcile-mounted, mechanically-fired weapons. However, I think we're really at a stage where we can seriously meet some of these constant weight-reduction requirements for the dismounted soldier, by displacing belt-fed 7.62 GPMG's, and DMR's, with a more capable cartidge, in a lighter system. Watch what it does across the target array on this range...."

It would appear that his vision would result in a 2-caliber system for the dismounts.

[stanc]

It would appear that his vision would result in a 2-caliber system for the dismounts.

His vision is for the operational use of two calibers by dismounted infantry.

But it would require adoption of a third rifle/machine gun caliber by the entire military force structure.

7.62 NATO will be around for a long time, and there is no need to get rid of it in aircraft and vehcile-mounted, mechanically-fired weapons.

- 5.56 NATO for carbines
- 6.5 Super G for LMGs and DMRs
- 7.62 NATO for vehicle and aircraft MMGs

I know of no historical precedent wherein the Army chose to add a new rifle caliber, without concurrently phasing out an old caliber.

However, there is historical precedent for refusing to adopt a third caliber: The 6mm SAW was proposed for a similar purpose as the 6.5 Super G, but rejected.

The other two concepts (6.5 Grendel, 6.X GPC) take the proven historical path, and would completely replace 5.56 NATO throughout the military.